One class of synchronous condensers is, as described in Ser. No. 08/242,890, now U.S. Pat. No. 5,610,500, essentially a large generator made for use as a turbine-generator, acting to improve the power factor of the power line or grid. As such, it serves a useful function, but it is not always needed, depending upon the power factor of the instantaneous load. Because the operation of the synchronous condenser consumes a certain amount of energy, approximately one percent of the rated MVA, it is desirable to be able to take it off line when it is not needed. Some synchronous condensers are designed with end windings of sufficient strength to be able to be started under the condition known as "across the line" start; wherein with the rotor not rotating, the contactor which connects the three phases of the motor or synchronous condenser is closed. At closure of the contactor, the connection out of phase, causes a corresponding large electro-magnetic force, which--produces a heavy torsional shock to the windings of the apparatus. These are not the synchronous condensers to which this invention is directed. The present invention has to do with turbine generators converted to synchronous condensers, or potentially, large synchronous condensers of a similar nature, that require an external starting mechanism. The end windings of the stator of such synchronous generators suffer most from the large electro-magnetic forces because the end windings are not supported as well as are the windings located in the axial slots in the stator core. Once up to operating speed, these electromotive forces are reduced.
In the case of the large generators made for service as a turbine driven generator, when used normally, the turbine is used to increase the speed to normal operating speed, and the rate of increase is very slow, from fifteen minutes to several hours. On the other hand, in an "across the line" start, it takes only 11 to 30 seconds to increase the speed from zero to full operating speed. Another reason, besides the torsional shock, for using a slow start for these large generators/synchronous condensers is related to the metallurgical properties of the rotor forgings. Many of them now in service and those that could be made in the future have alloy steel forgings with an alloy that acts more in a brittle manner at ambient or colder conditions, i.e. less than 70 degrees F., while at the normal operating conditions, i.e. above 120.degree. F., the alloy behaves in a more ductile manner. This property is described as the "Fracture Appearance Transition Temperature, abbreviated "FATT". The temperature at which the alloy acts 50% brittle and 50% ductile is referred to as the FATT, also known to some metallurgists as the Ductile-Brittle Transition (DBT). An explanation can be found in Marks' Standard Handbook for Mechanical Engineers, eighth edition, Mc Graw Hill, publisher, pages 5-6 through 5-9. Thus it is important to assure that the operating temperature of a generator/synchronous condenser is above the FATT before the rotor is operated at high speed, in order to assure that the material is acting in the ductile range before being subjected to high tensile stress from either (a) centrifugal forces or (b) high bending stresses associated with large amplitudes which can be encountered during high amplitude lateral vibratory conditions which occur when the rotor speed passes through the critical speed of the rotor bearing system.
Failures of generators in the past have been related to both of these factors. Large generator rotors have failed when started under cold conditions directly. Metallurgical examination identified operation at a temperature below the FATT to be the source of the later failure. Further, large generators with the rotor assembly at rest have inadvertently been connected electrically to the grid, causing damage to the generator windings, particularly the end windings.
In addition to the desirability of being able to bring up the speed of rotation of the synchronous condenser at a controlled rate, slowly relative to the rate at which an "across the line" start would be, it is desirable to be able to locate the synchronous condenser in any portion of the electrical distribution system where the benefits of a synchronous condenser are desired, to make the system self-starting, with an unmanned start and a remote start from a system operator's control board. These latter characteristics are similar to those of existing systems for gas turbine generators, and the application to the system of this invention of the electronic control technology for remote starting and synchronization of gas turbine generators to the power grid, will be apparent to those skilled in the art.
One of the objects of this invention is to provide a relatively inexpensive apparatus and method for bringing a large rotating synchronous condenser onto line.
Another object is to accomplish the task of bringing the synchronous condenser up to speed at a controlled rate.
Still another object is to permit starting of the synchronous condenser from a control station completely remote from the site of the synchronous condenser.
Other objects will become apparent to those skilled in the art in the light of the following description and accompanying drawings.